I don't use categories anymore for the contexts in Outlook but I tag all my Next Actions with the context (e.g. @Calls Philip about GTD Setup number XXX).
You can search in Reminders keywords/contexts to pick up the next stuff to do.

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While Outlook is the default productivity application where I work, it's only mandatory for email and calendar. I assume this would be the case for many organisations because task lists are generally a personal thing rather than being group-oriented.

A number of task management applications that can be used on mobile devices also provide a web interface, synchronization over the Internet and email-to-inbox functionality so they remain well-connected to whatever system is being used for email and calendar.

I'm using Appigo Todo (iPhone, iPad, Mac at home and web on a PC at work). Toodledo has a similar offering.

I don't use categories anymore for the contexts in Outlook but I tag all my Next Actions with the context (e.g. @Calls Philip about GTD Setup number XXX).
You can search in Reminders keywords/contexts to pick up the next stuff to do.